Squealing noise when in reverse

Check your timing belt.

Is the noise under the hood? or where?

If under the hood, most likely the engine has a bad motor mount. The engine will "twist" slightly to one direction when driving forward and "twist" in the opposite direction when you drive in reverse.

When it is twisted (in reverse) an engine pulley or fanbelt is scraping on some plastic part, or whatever, and making the noise.

If not under the hood, it may be one of the front brake pads that have a "worn brake pad" sensor. The sensor is an L-shaped piece of metal that scrapes the rotor when the brake pad backing gets too close to the rotor, hence worn brake pads.

If the noise is somewhere else, maybe another "expert" has a suggestion.

Please post more information... Thank you.